I recently had this code:

var concurrency = os.cpus.length;

function doWork(opts, cb) {
  cb(null);
}

var queue = async.queue(doWork, concurrency);

Things queued never called the callback because os.cpus is a function that takes 0 arguments. Thus os.cpus.length returns 0 and the queue never does anything. The correct code would be os.cpus().length, notice the ().

Maybe it would be a good idea to throw an error to alert the user that the queue was initialised with 0 concurrency and thus never will run anything.